1. Field of the Invention
This invention relates to an objective lens actuating device and an optical pickup and an optical disc recording/reproducing apparatus employing the objective lens actuating device. More particularly, it relates to an actuating device for actuating an objective lens at least in a direction along the optical axis of the objective lens, an optical pickup unit employing such objective lens actuating device, and an optical disc recording and/or reproducing apparatus employing such optical pickup unit.
2. Background of the Invention
Up to now, an optical disc 64 mm in diameter and capable of recording music signals continuing about 74 minutes, has been employed as a recording medium for the optical disc recording and/or reproducing apparatus. Such an optical disc is housed within a cartridge main body formed by upper and lower cartridge halves joined to each other to form a disc cartridge for protecting the optical disc during non-use time such as during storage and for achieving facilitated handling.
Thus the disc cartridge 2 has a cartridge main body 5 constituted by the upper and lower rectangular-shaped cartridge halves 3, 4 molded by injection molding of synthetic resin, as shown in FIGS. 1 and 2. Within the cartridge main body 5 is housed an optical disc 1. A metal plate 7 is attached to the optical disc 1 for covering its center opening. This metal plate 7 acts as a clamper attracted by a magnet provided on a disc table as later explained.
The lower half 4 of the cartridge main body 5 having the optical disc 1 housed therein has an opening 6 for the disc table of the rotational disc driving unit adapted for rotationally driving the optical disc 1. Into the opening 6 is intruded the disc table when the disc cartridge 2 is loaded in the disc recording and/or reproducing apparatus. The opening 6 is contoured for facing the inner rim portion, inclusive of the metal plate 7, of the optical disc 1 housed within the cartridge main body 5, as shown in FIG. 2.
The upper and lower surfaces of the cartridge main body 5, that is the upper and lower halves 3, 4, are formed with recording/reproducing apertures 8, 9, respectively, for exposing a portion of a signal recording area of the optical disc 1 housed therein to the outside along its radius and to an optical pickup unit for recording/reproducing information signals. These recording/reproducing apertures 8, 9 are rectangular-shaped and positioned at a mid position along the left-and-right direction of the cartridge main body 5 for extending from the position proximate to the opening 6 for the disc table as far as the front end face of the cartridge main body 5, as shown in FIGS. 1 and 2. The recording/reproducing apertures 8, 9 are opened and closed by a shutter 10 having a U-shaped cross-section which is fitted on the front end face of the cartridge main body 5. Thus, during non-use time of the disc cartridge 2, the recording/reproducing apertures 8, 9 are closed by the shutter 10. When the recording/reproducing apertures 8, 9 are closed the shutter 10 is locked by a lock member arranged within the cartridge main body 5 for holding the shutter 10 over apertures 8, 9 in the closed position.
The disc cartridge 2 is sized to hold an optical disc 1 that is 64 mm in diameter (R), as shown in FIG. 1. Specifically, the disc cartridge 2 is of a width W.sub.1 (along which the shutter 10 is moved for opening or closing the recording/reproducing apertures 8, 9, which is equal to 68 mm, a width W.sub.2 (perpendicular to the width W.sub.1 and equal to 72 mm), and a thickness D (equal to 5 mm).
Each of the recording/reproducing apertures 8, 9 formed in the cartridge main body 5 is of a length L.sub.1 (along the radius of the disc 1 and equal to 24 mm) and a width W.sub.3 (equal to 17 mm), as shown in FIG. 2.
By employing the above-described small-sized disc cartridge as the recording medium, it becomes possible to reduce the size of the optical disc recording and/or reproducing apparatus itself.
The optical disc recording and/or reproducing apparatus employing the optical disc as a recording medium has the optical pickup unit for condensing and radiating a light beam from a light source, such as a semiconductor laser, onto the signal recording area of the optical disc and for detecting the return light from the optical disc for recording information signals on the optical disc or reproducing the information signals prerecorded on the disc. Such optical pickup unit is comprised of optical components, such as an optical block made up of the semiconductor laser as the light source for radiating the light beam onto the optical disc, a photosensor for detecting the return light from the optical disc, a beam splitter for separating the light beam emanated from the semiconductor laser from the return light outgoing from the semiconductor laser, and an objective lens driving device having an objective lens for converging the light beam radiated from the light source to the recording track on the optical disc and for forcing the light beam to follow the recording track on the optical disc.
The objective lens driving device, constituting the optical pickup unit employed in the optical disc recording and/or reproducing apparatus, is comprised of a support base 11 constituting a magnetic circuit, a bobbin support 12 mounted in a cantilevered fashion on the supporting base 11 and a bobbin 14 carrying an objective lens 13, as shown in FIG. 3.
On the support base 11 are set a pair of supporting pins 16, 17 for supporting a stationary fixation portion 15 provided at the proximal end of the bobbin support 12. A pair of U-shaped yokes 18, 19 are formed at both distal sides of the support base 11. Magnets 20, 21 are mounted on the inner lateral sides of lugs 18a and 19a constituting the yokes 18, 19, respectively.
The bobbin support 12 has the fixation portion 15 molded from a synthetic material and having pin inserting holes 22, 23 that engage the supporting pins 16, 17, respectively. A pair of parallel supporting arms 25, 26 are extended to form a lateral side of the fixation portion 15. The supporting arms 25, 26 have their distal ends connected together by a connecting portion 12a. The connecting portions of the parallel supporting arms 25, 26 to the fixation portion 15 and the connecting portions of the supporting arms 25, 26 to the connecting portion 12a, are formed as deflecting portions 27, 27 and 29, 29 in the focusing direction extending parallel to each other along the width of the supporting arms 25, 26. These deflecting portions are of reduced thicknesses and are adapted for being displaced in the focusing direction. By forming the deflecting portions in this manner, the parallel supporting arms 25, 26 may be displaced parallel to the axes of the supporting pins 16, 17 supporting the fixation portion 15. At the distal ends of the connecting portions 29 is formed a bobbin mounting portion 32 via a deflecting portion 31 in the tracking direction. Bobbin mounting portion 32 is reduced in thickness and formed parallel to the axial direction of the supporting pins 16, 17 supporting the fixation portion 15.
The bobbin 14, supported via the bobbin mounting portion of the bobbin support 12, has a lens mounting portion 33 for mounting the objective lens 13 at its one end. The objective lens 13 is mounted in the lens mounting portion 33 via a lens holder fitted in an engaging opening in the lens mounting portion 33. The bobbin 14 also has a U-shaped cut-out 34 in which the parallel supporting arm 25, 26 are extended from the opposite end towards the center of the bobbin 14. On the inner lateral side of the cut-out 84 is formed a fitting recess 35 in which is fitted the bobbin mounting portion 32 provided at the distal end of the bobbin support 12. In the opening end of the cut-out 34 is mounted a balance weight for maintaining weight balance with respect to the objective lens 13 attached to the lens mounting portion 33.
On the opposite lateral sides of the upper surface of the bobbin 14 are formed coil setting recesses 37, 38. In these mounting recesses 37, 38 are set focusing coils 39, 40 wound in square-shaped tubes. On the outer lateral sides of the focusing coils 39, 40 are pairs of tracking coils 41, 42 wound in the form of flat rectangles.
The bobbin 14 carrying the objective lens 13 is supported by the bobbin support 12 by having the bobbin mounting portion 32 at the distal ends of the parallel arms 25, 26 in the fitting recess 35 formed in the inner lateral sides of the cut-out 34. The objective lens actuating device is completed by inserting the supporting pins 16, 17 through the pin inserting openings 22, 23 bored in the fixation portion 15 for having the bobbin support 12 carrying the bobbin 14 mounted on the supporting base 11. At this time, the legs 18b, 19b of the yokes 18, 19 provided on the supporting base 11 are introduced into the tubular focusing coils 39, 40 mounted on the bobbin 14, with the magnets 20, 21 attached to the lugs 18a, 19a facing the focusing coils 39, 40 and the tracking coils 41, 42.
In the above-described objective lens driving device, a driving current proportional to the focusing error signal is supplied to the focusing coils 39, 40. Such driving current cooperates with the magnetic flux produced by the magnets 20, 21 to generate a driving force in the focusing direction parallel to the optical axis of the objective lens 13. By such driving force, the parallel arms 25, 26 are deflected along the focusing direction shown by arrow F in FIG. 3, with the deflecting portions 27, 27 and 29, 29 as the points of deflection. As a result, the objective lens 13 mounted on the bobbin 14 supported on the distal ends of the parallel arms 25, 26 is deflected in the focusing direction for effecting the focusing control.
A driving current proportional to the tracking error signal is supplied to the tracking coils 20, 21. Such driving current cooperates with the magnetic flux produced by the magnets 20, 21 to generate a driving force in the tracking direction normal to the optical axis of the objective lens 13. By such driving force, the bobbin 14 supported at the distal ends of the parallel arms 25, 26 is deflected along the tracking direction shown by arrow T in FIG. 3, with the deflecting portion 31 as the point of deflection. As a result, the objective lens 13 is deflected in the tracking direction along the radius of the optical disc normal to the optical axis and, the light flux is caused to follow the recording track in the optical disc for effecting the tracking control.
With the above-described objective lens driving device, the fixation portion 15 provided on the proximal end of the bobbin support 12 is supported by the supporting base 11, and the bobbin 14 carrying the objective lens 13 is supported via the bobbin mounting portion 32 mounted at the distal ends of the parallel supporting arms 25, 26 extended from the fixation portion 15. The magnetic circuit is made up of the yokes 18, 19 and the magnets 20, 21 for generating the driving force for displacing the objective lens 13 mounted on the bobbin 14 in the direction parallel to the optical axis of the objective lens 13 and in the direction normal to the optical axis along with the focusing coils 39, 40 and the tracking coils 41, 42 mounted on the bobbin 14.
The magnetic circuit is arranged between the stationary fixation portion 15 to the supporting base 11 and the objective lens 13. Consequently, the parallel supporting arms 25, 26 carrying the bobbin 14 are increased in length and the length between the mounting portion 15 and the bobbin 14 is increased, resulting in the increased length of the objective lens actuating device.
If the above-described large-sized objective lens actuating device is applied to the optical disc recording and/or reproducing apparatus employing the disc cartridge 2 housing the optical disc 1 (64 mm in diameter), the bobbin 14 including the objective lens 13 only partially faces the recording/reproducing apertures 8, 9, with the remaining portion of the bobbin extending on the lower surface of the cartridge main body 5, as shown in FIG. 4.
Thus the objective lens actuating device needs to be mounted at a position spaced some distance apart from the lower surface of the disc cartridge 2 loaded on the optical disc recording and/or reproducing apparatus, in order that the bobbin 14 or the bobbin support 12 is not contacted with the rim of the recording/reproducing apertures 8, 9 of the cartridge main body 5 when the objective lens 13 is displaced along the optical axis.
If the objective lens actuating device is arranged in this manner at a position spaced some distance away from the disc cartridge 2, the objective lens 13 for focusing the light beam in the signal recording region of the optical disc 1 is increased in focal length, as a result of which the objective lens 13 and hence the objective lens actuating device are increased in size. Thus it becomes difficult to reduce the size of the optical disc recording and/or reproducing apparatus employing the objective lens actuating device.
Thus, attempts have been made in decreasing the differential distance between the end face of the objective lens 13 and the signal recording area of the optical disc 1, and in employing a small-sized objective lens 13 with a small focal length in order to reduce the size of the objective lens actuating device. One such objective lens actuating device is shown in FIG. 5, and includes a lens supporting lug 43 protrudes from an end of the upper surface of the bobbin 14, and the lens mounting portion 33 is mounted on this lens supporting lug 43, by means of which the objective lens 13 is mounted. This permits the objective lens 13 to face the inside of the recording/reproducing apertures 8, 9 and to approach to the optical disc 1. In addition, the differential distance between the objective lens 13 and the disc 1 is diminished to render it possible to employ the small-sized objective lens 13 to reduce the size of the objective lens driving device.
However, even if the objective lens 13 and hence the objective lens driving device are reduced in size, the portion of the bobbin other than the objective lens 13 is extended along the lower surface of the cartridge main body 5, so that it is difficult to further reduce the size of the optical disc recording and/or reproducing apparatus.
In addition, if the objective lens 13 is mounted via the lens mounting lug 43, the lens supporting lug 43 or the supporting arms 25, 26 are deformed significantly with lapse of time. Consequently, the optical axis of the objective lens 13 is skewed and hence its optical axis cannot be maintained with high accuracy so as to be normal to the optical disc 1.
Besides, since only the objective lens 13 is protruded from the bobbin 14 for approaching the optical disc 1, the objective lens 13 is mounted at a position spaced apart from the magnetic circuit generating the driving force, so that it becomes difficult to deflect the objective lens 13 with high response to the driving force proportionate to the focusing and tracking signals.
On the other hand, it becomes impossible to effect accurate focusing control and tracking control of the objective lens 13, so that recording and/or reproduction of information signals occasionally cannot be achieved with good recording/playback characteristics.